Field of the Invention
The present invention relates to a honeycomb body, in particular a catalyst body for automobiles, including a casing tube being conical relative to an axis, at least one coiled stack disposed in the casing tube and having a multiplicity of metal layers disposed on one another, at least some of the layers at least partially having waves, and a plurality of channels through which a fluid can flow.
Such a honeycomb body is known from International Publication WO 93/20339, corresponding to U.S. Pat. No. 5,506,028. That publication describes a honeycomb body with an axis and with a casing tube which is conical with respect to the latter and into which is fitted a configuration composed of at least one stack coiled around the axis in an involute manner. The stack has a multiplicity of metal layers disposed on one another.
Each layer is shaped in the manner of an annular segment, so that it is limited by an outer arc which is approximately circular with respect to a center point, and by an approximately circular inner arc that is concentric to the outer arc and located between the latter and the center point. Each corrugated layer has waves. The corrugation of a layer does not have a constant wave height over the entire layer. The wave height must increase, ranging from a smaller wave height on the smaller arc limiting the layer to a greater wave height on the larger arc limiting the layer. In that case, the ratio of the wave heights must correspond approximately to the ratio of the lengths of the arcs, so that an approximately conical honeycomb body is obtained when the layer is coiled.
A honeycomb body, as described in International Publication WO 93/20339, corresponding to U.S. Pat. No. 5,506,028, is suitable, in particular, as a carrier for a catalyst in order to bring about a catalytic reaction in a fluid flowing through it. It is suitable, in particular, as a precatalyst for a honeycomb body of a known type, wherein the conical honeycomb body is disposed in a diffuser of the exhaust system immediately upstream of the known honeycomb body. Since the conical honeycomb body serves as a diffuser for the downstream honeycomb bodies which are known per se, a uniform onflow to a following honeycomb body should be achieved. The conically constructed honeycomb body may also be disposed downstream of the honeycomb body, so that it acts as a confuser. The problem of a uniform onflow to a honeycomb body carrying a catalyst is described in European Patent 0 386 013 B1.
In a honeycomb body of the generic type, the structured layers form a multiplicity of channels or ducts, through which a fluid is capable of flowing. In conventional dimensioning, the flow of a fluid in the channels is essentially laminar, since the channel cross section is relatively small. As a result, relatively thick boundary layers form on the channel walls and reduce the contact of the core flow in the channels with the walls. A reduction in the contact of the core flow with the walls leads, under some circumstances, to a reduced catalytic effect of the honeycomb body provided with a catalyst.
European Patent 0 484 364 B1 discloses a honeycomb body, in particular a catalyst carrier body, composed of at least partially structured metal layers which form the walls of a multiplicity of channels, through which a fluid is capable of flowing. In that honeycomb body part of the layers have a main corrugation with wave crests and wave troughs and with a predeterminable wave height. The wave crests and/or wave troughs are provided with a multiplicity of turned-over portions, the height of which is smaller than or equal to the wave height, with the result that channels having additional onflow edges are formed inside. By virtue of that structure of a honeycomb body, which acts as a main catalyst, a higher catalytic conversion rate is achieved, with the same use of material, than in the case of corresponding bodies without any turned-over portions.
Furthermore, European Patent 0 152 560 B1 discloses a honeycomb body in which the corrugations of a metal layer form flow channels that are disposed one behind the other in the direction of flow, but so as to be offset relative to one another transversely thereto. The flow channels are formed by corrugated strips which are provided alternately with wave crests and wave troughs and are directly contiguous with one another at their front and rear edges running transversely to the direction of flow. They are offset relative to one another in each case by a fraction of their wavelength and form an interconnected layer strip. By virtue of that structure of the layers, an increase in turbulence is also achieved in the radial direction within the honeycomb body through which the flow passes. That results in an equalization of the flow profile and in action on the edge zones of the honeycomb body which consequently participate in the reaction and. thereby increase the reaction effect of the honeycomb body.